Electric motors of this kind—frequently also known as permanent magnet excited motors—are well-known; they are characterized by a high degree of efficiency and are suitable—in particular in an embodiment as an external rotor motor—as drive mechanisms for fans and blowers. Owing to magnetic conductance fluctuations along the gap which are caused by the grooves of the iron core of the stator, usually of the stator lamination stack, torque fluctuations, known as cogging torques, occur during the operation of these motors which disadvantageously can result in vibrations and noise. In the particular case of fans which are used in refrigeration and air conditioning, however, low-noise operation with a high degree of efficiency is required.
Some measures for reducing the cogging torques in permanent magnet excited motors are known from the prior art. For example, DE 101 47 310 B4 describes cup-shaped magnet segments with oblique sides arranged in the axial direction. Due to the likewise obliquely arranged pole transitions, the cogging torque is reduced here. At the same time, however, the permanent magnetic flux linkage and thus the torque density and the degree of efficiency are decreased in comparison to an embodiment with straight transitions. Furthermore, the production of such magnet segments is associated with additional costs.
Magnet segments with axially oblique sides are also known in the other documents DE 199 01 310 A1, DE 295 10 938 U1, JP 561 53 961 A as well as in EP 0 375 228 B2 and/or U.S. Pat. No. 4,933,584.
Documents EP 0 375 228 B2 and/or U.S. Pat. No. 4,933,584 describe rectangular or semi-circular concave recesses at the stator teeth. The recesses discontinuously merge into one another with steps and edges in the circumferential direction. The recesses are additionally arranged obliquely in the axial direction as well as the slots. In addition, the magnetization of the permanent magnets is implemented obliquely in the axial direction. This is known to reduce not only the cogging torque, but also the torque density and the degree of efficiency of the motor. In particular an oblique slot is also associated with an increased expense for introducing the winding. Furthermore, the beveling here also results in the reduction in permanent magnet flux linkage and thus to a reduction in torque.
Document JP 56 153 961 A describes recesses at the teeth with the objective of reducing the cogging torque, however, with uneven slot division (tooth widths). Thus, primary and auxiliary teeth are produced. The recesses have the shape of concave semicircles and discontinuously merge into one another with steps and edges in the circumferential direction. With respect to the central axes of the slots, the recesses at the teeth are asymmetrical. Furthermore, the magnetization of the permanent magnets runs obliquely and/or offset in the axial direction. A beveling of this kind is known to reduce the permanent magnet flux linkage and thus the torque density and the degree of efficiency of the motor.
The other documents DE 28 50 478 C3, EP 0 081 524 B1, EP 0 473 534 B1 and EP 0 545 060 B1 respectively describe permanent magnets with straight sides, but oblique pole transitions owing to the corresponding magnetization. This embodiment likewise has a negative effect on the torque density and the degree of efficiency. In addition, an oblique pole transition has still another disadvantage. The actually favorable angle of inclination for reducing the cogging torque depends on the axial length of the stator and/or stator iron core (stator lamination stack), so that separate magnetization devices have to be available for each different length.
Document DE 37 10 658 A1 relates to a dc motor with a rotor featuring multi-part permanent magnets in the axial direction, wherein the individual parts are arranged offset to one another in the circumferential direction. This arrangement basically also results in the reduction in cogging torque, but at the expense of the torque density and the degree of efficiency as well as in connection with increased manufacturing expenses for installing the partial magnets and their magnetization.
Document DE 37 23 099 C2 describes an embodiment with a gap which is periodically different in the circumferential direction formed by axial, magnetically conducting elevations in the form of cams on the stator teeth. In this case, the cams protrude into the gap. Under consideration of manufacturing tolerances, the overall size of the gap must be designed larger in order to ensure a sufficient gap in the region of the cams as well. As a result of this, however, the torque density and the degree of efficiency are reduced, and/or increased manufacturing expenses are required to achieve greater accuracy.
Finally, DE 103 03 848 A1 describes an asymmetrical arrangement of the magnets around the circumference of the rotor for the reduction of the cogging torque. This solution is associated with additional manufacturing expenses for the exact positioning of the magnets. Furthermore, the asymmetry resulting in this way has an adverse impact on the electromagnetically produced torque in the form of additional alternating torques in comparison with a symmetric arrangement.